WATER TURBINES

187,

best peripheral speed lies between 0-44 and 0-48 V^ the higher ratio being
possible with the most efficient buckets.

Fig, 22

Specific Speed. — As for a reaction turbine, the specific speed of a Pelton
wheel is given by

N -
~~

But N = — , where D is the diameter of the pitch circle,
TrD

and P = - L—!- — V^;, where d is the diameter of the jet
4

= o-6io^2H" if the efficiency n = 0-85*

Also u

(approx.).

xr _ 60 x o-^6s/2gH. x

/. JN,

If D -T- d = 12, this makes the specific speed approximately 4-6. This
is the highest value of the specific speed for a single-jet Pelton wheel for high